The Risk Register assessed as red the "High Level Risk" that the ONR is unable to train and warrant an adequate number of new inspectors in time.

Separately, a failure to "recruit enough people with the right skills to deliver a UK State System of Accountancy for control of nuclear material to meet international obligations by 2019" was also a red risk.

Image: The ONR said 'good progress' had been made

The document lists seven ways that risk can be mitigated, including "redefined person spec" and that standards may be lowered to plug the skills gap, suggesting there are insufficiently trained personnel.

The Business Department did say in a recent update to parliament that 13 inspectors and trainee inspectors had been recruited.

Parliamentary sources have raised some doubt that they could be trained in time, claiming it takes five years to train a safeguards inspector fully.

Image: Under a no deal Brexit, the legal basis of nuclear trade would disappear

The key underlying risk of not being prepared for Brexit day is identified by the Risk Register as a lack of secure funding.

Subsequently, the Business Department has released £8m in two tranches to fund this activity, but industry sources are still highly concerned that the systems will not be in place.

But if Britain quits the EU without a deal and IT systems are not ready, the legal basis of trade in nuclear material, parts and expertise would disappear.

Nuclear operators have warned of shutdowns to nuclear power stations, and the transfer of crucial medical isotopes being blocked in such a scenario.

Most industry sources are pinning their hopes on an agreed transition within the Euratom system, as outlined in March by the EU's chief negotiator Michel Barnier and UK Brexit Secretary David Davis.

Rachel Reeves, chair of the Commons' influential business select committee, said the Government's plans in such a short time-scale were "going to be incredibly challenging, if not impossible".

Image: Ms Reeves said nuclear cooperation did not have to mean betraying Leave voters

"And if there is any risk at all that leaving Euratom is going to leave us with a less robust regulatory regime or not being able to import and export the things we need to keep the lights on, the Government need to face into that and put in place proper robust plans to do so."

She added: "I think we can have cooperation in the area of nuclear energy that doesn't defy the will of the people."

Tom Greatrex, chief executive of the Nuclear Industries Association, also told Sky News: "If no deal means no transition then this is a very exacting time frame to try and do everything by.

"I'm not at all confident that we can get everything done by March 2019."

The ONR is meeting with the International Atomic Energy Agency in June and September to decide if the new UK organisation can be a certified nuclear regulator.

A spokesperson for the ONR told Sky News: "Good progress has been made on the procurement of the new IT system and on the recruitment and training of new safeguards inspectors.

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"We are confident that these will be in place to ensure we meet international obligations for day one of exiting Euratom."